1. Field of the Invention
The present invention relates to a method and an apparatus for controlling the rotation of one of a plurality of rotary support bodies supporting an endless belt and to which drive torque is transferred, and an image forming apparatus including the same.
2. Description of the Background Art
An electrophotographic image forming apparatus of the type including a photoconductive belt, intermediate image transfer belt, sheet conveying belt or similar endless belt is conventional. The prerequisite with this type of image forming apparatus is that the drive of the belt should be accurately controlled in order to insure high image quality. Particularly, in a tandem, color image forming apparatus feasible for a high speed, small size configuration, a belt for conveying a sheet or recording medium must be driven with high accuracy. More specifically, in a tandem, color image forming apparatus, and endless belt conveys a sheet via a plurality of image forming units arranged side by side in the direction of conveyance and assigned to different colors. In this condition, toner images of different colors are sequentially transferred to the sheet one above the other, completing a color image.
In a specific configuration of the tandem, color image forming apparatus, a yellow, a magenta, a cyan and a black image forming unit are sequentially arranged in this order in the direction of sheet conveyance. The yellow to black image forming units each develop a toner image formed on a particular photoconductive drum by a laser scanning unit, thereby forming a toner image. Such toner images are sequentially transferred one above the other to a sheet being conveyed by a belt while being electrostatically retained on the belt, completing a color image. Subsequently, a fixing unit fixes the color image on the sheet with heat and pressure.
The above belt is passed over a drive roller and a driven roller, which are parallel to each other, while being subject to adequate tension. The drive roller is driven by a motor at preselected speed and causes the belt to turn at preselected speed. The sheet is conveyed to the image forming unit side of the belt by a sheet feed mechanism at preselected timing. The sheet is then conveyed via the consecutive image forming units at the same speed as the belt.
In the tandem, color image forming apparatus of the type described, it is extremely important to cause the a sheet, i.e., the belt to move at preselected speed, so that the toner images of different colors can be superposed on the sheet in accurate register.
To accurately control the drive of any one of different kinds of endless belts mentioned earlier, it is a common practice to cause the drive roller to rotate at constant speed by maintaining the angular velocity of the motor or that of a gear meshing with the drive roller constant. This control scheme, however, cannot maintain the belt speed constant if the thickness of the belt is not constant, particularly in the direction in which the belt moves.
To solve the above problem, Japanese Patent No. 2,639,106, for example, proposes to control the rotation speed of a drive roller by measuring the thickness of a belt beforehand and then calculating the parameter of a drive source, which is necessary for maintaining the belt speed constant, on the basis of the thickness. However, this scheme is difficult to practice because it is extremely difficult to measure the fine thickness of a belt. Further, although no extra part cost is required, measured data must be input in the apparatus on the production line or the market, increasing production cost and service cost.
Japanese Patent Laid-Open Publication No. 2001-228777 proposes to correct the rotation speed of a drive roller while measuring the thickness of a belt or to record the thickness variation of the belt over one turn and then correct the above rotation speed on the basis of the thickness variation. This proposal, however, has a problem that it is extremely difficult to effect real-time measurement of fine belt thickness and a problem that production cost increases because an expensive sensor, for example, is necessary for enhancing sensitivity.
Further, Japanese Patent Laid-Open Publication No. 2000-310897 teaches a control scheme pertaining to a belt formed by centrifugal molding and apt to vary in thickness over one turn in the form of a sinusoidal wave. In accordance with this control scheme, before the belt is mounted to an apparatus body, the thickness profile or irregularity of the belt is measured over the entire circumference on the production line and written to a ROM (Read Only Memory). Subsequently, a reference mark representative of a home position is provided on the belt at a position where the thickness profile over the entire circumference appears in the same phase. By detecting the reference mark of the belt, it is possible to control belt drive means in such a manner as to cancel the speed variation of the belt ascribable to thickness variation. However, this control scheme is not practicable without noticeably increasing cost necessary for the production of the belt.
Japanese Patent Laid-Open Publication No. 22-174932 teaches that by storing a relation between a control target and errors occurred during past operation and then correcting the control target, it is possible to maintain the movement of a belt more stable against thickness variation (see paragraph 0034). This document, however, does not describe the correction of the control target or control specifically.